As is well known, in an impact dot printer, a printing hammer or wire forces an inked ribbon to strike a printing paper at a quite high velocity to perform printing. During this operation, the ribbon and the paper rub against each other momentarily and repeatedly, producing static electricity on the ribbon. In case of a ribbon cassette, the casing is made of plastic, and therefore the static electricity generated on the ribbon can escape to nowhere, thus tending to build up. Eventually, a discharge will occur between the ribbon and the front surface of the head, the printing hammer or wire. This sometimes may cause a malfunction of the printer.